If you have in and out as well as fore and aft movement, the box needs rebuilding. The adjustment screw changes the depth of engagement between the worm and follower. To adjust the depth, jack up the front end to get the wheels off the ground and tighten the screw down until you have no lost motion when you rock the wheel back and forth. Then, turn it lock to lock to make sure you haven't gotten too tight at the extremes. If the box tightens up in a turning maneuver, back off the screw a tad until it is adjusted properly in the full lock position. It is normal for a box with some wear to be a little loose with the wheels straight ahead, and tight as you get farther into a turn movement. This is normal wear, and shouldn't be anything to concern you. Up and down movement is caused by worn bushings or bearings and must be corrected before adjusting the box.
This is standard procedure for all the old truck and car steering boxes that date to that time period.
